Internal nutrient loading from large lake sediments: revisiting legacies and paradigms

Session: Harmful Algal Blooms: From Ecosystem Drivers to Ecosystem Impacts (3)

Mark McCarthy, Wright State University, [email protected]
Silvia Newell, Wright State University, [email protected]
Justin Myers, Wright State University, [email protected]
Wayne Gardner, UTMSI, [email protected]

Abstract

When scientists, resource managers, and regulators discuss ‘internal loading’ or ‘legacy nutrients’, they are usually talking about phosphorus, not nitrogen, accumulated in sediments. Indeed, the best natural defense mechanism that a lake has against nitrogen pollution is a microbial process, denitrification, occurring in those sediments. Denitrification (and anammox) removes reactive nitrogen from the system before it can be recycled internally. However, modern harmful cyanobacterial blooms are increasingly associated with non-nitrogen-fixing, nitrogen-rich toxin producing taxa (e.g., Microcystis and Planktothrix), especially during warm, low runoff seasons (mid-late summer) in some areas. We can thus no longer disregard internal nitrogen loading as a potential driver of these blooms. Like internal loading from legacy phosphorus, the sediment nitrogen pool is ultimately derived from external sources, such as agriculture, domestic wastewater, and urban runoff. High nitrogen concentrations can sometimes even enhance internal phosphorus loading. Internal phosphorus (as orthophosphate) and nitrogen (mostly as ammonium) loading from sediments have been measured using continuous-flow incubations of intact sediment cores in multiple large lake systems around the world. We present results from several eutrophic, cyanobacteria bloom-impacted lakes showing that internal sediment nitrogen loading in these lakes far exceeded that of phosphorus, even after considering nitrogen removal via denitrification.